**2. Deposition Technologies and Coating Systems**

Various deposition technologies have been used to synthesise (CrAl)N and (AlCr)N coatings over a period of about 30 years. Basic information about the deposition technologies is beyond the scope of this review. The interested reader is invited to look at available publications [24–31]. Some more exotic evaporation methods are hollow cathode arc evaporation [16] and activated reactive evaporation [32]. The first application of AlCr-based coatings for cutting tools deposited by hollow cathode arc was mentioned very early [16,33]. However, primarily sputtering processes and arc processes were used both for basic investigations as well as for applications.

The first reported (CrAl)N coatings were DC (Direct Current) magnetron sputtered [14,15]. The first arc coatings were deposited using elemental cathodes of pure Al and pure Cr [17]. The first multilayer coating (AlCr)N was (AlCr)N/CrN deposited by arc [19]. Sometimes even hybrid methods, such as arc plus hollow cathode discharge [34] or, more commonly, arc plus sputtering [35], have been used. Alloyed CrAl-based and AlCr-based coatings are most commonly deposited either by magnetron sputtering or by arc evaporation. For sputtered coatings, the preferred methods are various HiPIMS processes in industrial applications, e.g., the S3p process [30]. The deposition of alpha alumina coating is based on a dedicated pulsed arc process, the P3e process [23].

It should be mentioned that the main differences between arc deposition processes and sputtering are:

**Growth defects:** Arc-deposited coatings have more growth defects in the coating than sputtered coatings due to macroparticle generation in the arc evaporation process. Sputtered coatings are thus preferred in applications requiring particularly low surface roughness.

**Growth rate:** The arc process allows a higher growth rate to be achieved than sputtering, depending on the PVD system set-up.

**Degree of ionisation and ion energies:** Arc evaporation is known to produce plasmas with a high degree of ionisation. Multiple charge states are formed as well in arc evaporation [36]. Sputtering processes are characterised by lower degrees of ionisation [26].

**Energy consumption:** The specific energy consumption of the arc process is lower than that of sputtered coatings for the same coating thickness [37].

**Process stability:** Arc processes tend to exhibit higher process stability than sputtering processes.

**Larger coating volumes**: Arc processes can be more easily upscaled for coating long parts (several metres).

The different basic methods used for CrAl-based and AlCr-based coatings are listed here briefly.
